1. Field of the Invention
This invention is related generally to the field of gas fired turbines and more particularly to apparatus for prevention of icing in such turbines in the presence of low ambient temperatures.
2. Prior Art
A gas fired turbine draws in fresh air, which is compressed, and then fed into a combustion chamber which provides combustion gas to drive turbine blades. In a cold environment, when ambient temperatures drop below +35.degree. F., the air inlet to the turbine may freeze up due to condensation of water vapor in the air, blowing snow or ice fog. To overcome this problem, some means of anti-icing are essential. In accordance with a typical prior art anti-icing system, a recirculation line interconnecting the air inlet and the exhaust gas stack permits enough hot exhaust gases to be constantly recirculated to the inlet to maintain turbine inlet air temperature at a minimum of +40.degree. F. A bypass valve is positioned in the exhaust stack channel and dampers are provided which can close off the recirculation line. When anti-icing is not needed, the recirculation dampers are closed and the exhaust bypass valve is opened. When anti-icing is needed, the recirculation dampers are opened and the exhaust bypass valve is closed sufficiently to create a back-pressure which forces exhaust gases back to the turbine inlet.
The problem with this prior art system is that when anti-icing is not required, the exhaust gas is allowed to occupy the recirculation line between the exhaust stack and the recirculation dampers. As a result, moisture in the exhaust gas condenses out and accumulates around the recirculation line dampers, causing the dampers to freeze in a closed position. The result is that the dampers are inoperable when needed. To avoid this possibility, gas turbine operators have been forced to constantly recirculate enough exhaust gas to maintain the inlet air temperature at +40.degree. F. so as to prevent the recirculation dampers from freezing. This solution has other drawbacks, including loss of horsepower, constant recirculation of corrosive exhaust gas and a hotter hot-gas path in the turbine. Furthermore, with constant recirculation of exhaust gases, so much moisture is present at the air inlet that when ambient temperature drops below -40.degree. F., condensation may cause the turbine to freeze up on the inlet unless constant attention is devoted to avoidance of this condition.